Jonty Harrison’s latest CD,
Voyages, is now out. The
liner notes include a very
brief note describing how
this all came about and the involvement of the
Ambisonic Toolkit. The big question being,
“How to successfully ‘down-mix’ Harrison’s (very-)multi-channel Espaces cachés
and Going / Places to two channel stereo for CD release?” You know the answer
to this one: Use the
ATK!

Some (Spatial) problems and (Spatial) solutions

The big problem: Espaces cachés and Going / Places are composed for large
loudspeaker arrays. (32 channels!) That’s great if you’ve got a big room like a
concert hall to fill. But what about squeezing down to stereo?

Sensibly, the speakers can be be grouped into ‘spatial stems’ according to
their role in the pieces. E.g., of the 32 channels the array for
Espaces cachés, there are three 8 channel sub-groups: distant, main,
and close. In concert, the speakers for each of these sub-groups are placed in
the actual space accordingly. For the CD mix, we’ve got to somehow reproduce
this impression. But, just modeling the space (via designed or measured RIRs)
isn’t going to do the right thing. That would end up sounding like ‘adding
reverb’ to the mix… not good!

The answer, of course, is to model the spatial impression, instead. For each
‘spatial stem’, we apply a different spatial filter set. The processing for the
above sub-groups can be summarized as:

distant: diffusion filtering

main: mid-field proximity filtering

close: near-field proximity filtering

Details are slightly more complicated, of course, but the idea is to use the
ATK’s spatial filtering / processing to express the intended role of each
spatial stem. If we’ve got it right, we should hear the layering intended by
the composer in the UHJ stereo mix. (Yeah… we got it right!)

How to Listen?

By design,
Ambisonic UHJ Stereo is
stereo compatible. Listening
over your own stereo system, it’ll sound great. (This is one of the aspects of
working with Ambisonics I find so convenient, auto-magic stereo down-mix.) But
what about hearing Harrison’s
Voyages in full surround-y
gloriousness?

Here’s a very simple example from the
Help for
ATK for SuperCollider. You
won’t exactly want to do it this way because the below code loads the complete
file into memory. Instead, use
DiskIn.ar to stream in audio
from a file.

As a side note, you’ll be interested to hear that all the processing was done with the
vapour-ware HOA version of the ATK. (We should really call it the
super secret development version.) The mix was made in NFC-HOA5.